Signal transmission through optical fibers has become, or is becoming, the dominant signal transmission mode. The bandwidth characteristics of optical fibers, as well as their relative immunity to certain types of interference and contaminants make optical fibers the desirable transmission medium in high capacity trunk lines as well as in lower capacity feeder and distribution lines.
No matter what the intended end use may be, individual optical fibers generally are combined in an optical fiber cable which contains a plurality of such fibers, each of which is protected by at least one layer of coating material. In one configuration, the fibers are assembled into groups which are held together by binder ribbons or tubes to form a cable core. This is generally enclosed in a metallic or plastic tube or jacket which, in the latter case, often contains one or more strength members, typically of metal, such as heavy gauge wire. In another configuration, the fibers are arrayed in ribbon form and the core tube contains one or more stacked ribbons as well as strength members if desired.
Regardless of the cable configuration, it is usually necessary that the lengths of fiber cable be spliced at their ends to the ends of other cables, which entails splicing each of the individual fibers in a cable to a corresponding individual fiber in the second cable. To this end, there is provided a splice closure which usually comprises a protective case which contains at least one splice tray which, in turn, has a plurality of splice holders mounted thereon, into which the encased individual fiber splices are inserted and held. The cables are entrant into the case and generally are clamped to each end thereof to reduce the effects of tensile forces on the cables and on the splices. In U.S. patent application Ser. No 08/847,214 now U.S. Pat. No. 5,862,290 of Burek et al. (Burek Case 15-11) there is shown an optical fiber cable splice closure, generally for use outdoors such as in manholes or overhead cabling.
Inasmuch as, at the splice point, the cable itself is opened up and the base fibers are exposed, the only protection afforded the fibers is provided by the closure, which can provide only one or two layers of protection from the outside environment, and the requirements therefor are more stringent than for the cable, which normally provides several layers of protection. The closure must anchor the cables stored therein, and it must be capable of withstanding torsional and axial loads transmitted by the cable to the closure so that the splices are protected from these loads. The closure must also seal the inner and outer sheaths of the cables and maintain the seal integrity under extreme environmental conditions. In addition, the closure must provide adequate fiber storage for slack fiber without damaging the fibers and without increasing signal attenuation. The closure preferably should be capable of storing any type of splice, such as, for example, discrete mechanical, discrete fusion or mass fusion, or other types while reducing forces that tend to damage the splices. Additionally, the closure should provide adequate grounding and anchoring for the metallic strength members of the cable. The closure should also be capable of accepting high fiber count cables as well as those of low fiber count.
In order to insure protection of the splices from moisture, it is current practice to form the closure out of two mating halves, a base and a cover, with a grommet therebetween, and clamp them together. Cable entry is through openings in the grommet, which are usually supplied with inserts which seal the cable and in turn are sealed by the grommet. Such a grommet and insert arrangement is shown, for example, in U.S. Pat. No. 5,472,160 of Burek, et al.
Cables entrant into the enclosure are preferably, and in present day usage, almost always anchored to the splice enclosure itself, to guarantee a minimum of movement of the cable within the enclosure which could unduly stress the fibers and the fiber splices. One such anchoring means, in the form of a cable grip block, is shown in the aforementioned Burek, et al. U.S. Pat. No. 5,472,160. The grip block of that patent is capable of adapting to cables of different sizes, and also provides an anchor for the cable central strength member which is a usual component of loose tube type cables. The anchoring arrangement for the cable sheath strength member, a usual component of loose tube and unit tube cables, requires that the strength member or members be cut to a specific length and bent upward into a slot within the grip member. The strength members are maintained within the slots against tensile forces because of their rigidity and because they are bent at a right angle. Thus, they function to help maintain the cable against shifting or movement. The strength members, as pointed out hereinbefore, are preferably of metal and are typically grounded by connection to a ground bolt and lug which passes through a wall of the closure to the outside thereof. The lug, in turn, is connected to earth ground, and is sealed to the outside of the wall of the closure.
As is shown in the aforementioned Burek et al. application and the Burek et al. patent, individual splices are mounted on (or in) splice trays which can be stacked to accommodate up to 144 splices. With the large increases in optical fiber usage and in optical signal transmission, and with space limitations, the trend has been toward increasing the fiber splice capacity of splice closures. One arrangement for producing increased splice capacity relies upon increasing the number of splice trays in a stack in such closures as those of Burek et al. Other arrangements for stacking trays or equivalent splice holders have been developed commercially which are adaptable to the two pedestal splice tray holding arrangements of the Burek et al. closures. Thus, the so-called Ditel Pedestal Plate can be mounted on the dual pedestals of the Burek et al. enclosure, and the splice trays stacked upon a threaded rod extending from the pedestal plate. With either the Ditel or Burek et al. arrangement, the splice trays can be stacked to the extent that up to 432 discrete fiber splices may be mounted or held.
Necessarily, these stacks require an increased amount of vertical room, which is achieved by extending the splice closure cover. However, an extended cover is unavoidably weakened, or made less resistant to outside forces such as, for example, water pressure in a manhole. It is not uncommon for manholes to contain large amounts of water, even as much as a twenty foot stand of water in a thirty foot deep manhole. When the closure with the extended cover is at the bottom of such a manhole, the twenty foot stand of water can exert enough force on the cover to collapse it, or at least to bend it severely, thereby possibly destroying the water-tight integrity of the closure. Thus, it is highly desirable that the splice closure and cover therefor be so constructed as to resist these outside forces so that the interior components of the closure are protected from the deleterious effects of such forces such as, as previously mentioned, the destruction of the water-tight integrity of the closure.